I. Field of the Invention
This invention relates to a telephone line power utility circuit, and in more detail to a telephone line power utility circuit capable of utilizing a direct-current power supplied from the telephone lines as power sources for line terminal equipment such as telephone sets, modulation-demodulation devices (modems),facsimile devices and the like.
II. Description of the Prior Art
FIG. 7 is a circuit diagram of the conventional telephone line power utility circuit 501. In this telephone line power utility circuit 501, a direct-current voltage, which is supplied by the telephone line through a bridge diode BD, a hook switch HS, a DC separation transistor Q1 and a low-pass filter LPF, is converted by a DC-DC converter CN and used as a power source for a line terminal device main-body circuit TA. Symbol Vcc stands for a battery or the other external power supplies to be a power source of the line terminal device main-body circuit TA when the output from the DC-DC converter CN is insufficient.
A signal component is processed by being taken into a signal processing circuit (not shown in the figure) of the line terminal equipment main-body circuit TA through a direct-current interrupting capacitor C1 behind the hook switch HS.
A drive output circuit DO pulse-drives a switching element Tr1 of the DC-DC converter CN at a predetermined duty ratio.
FIG. 8 is a schematic diagram of a direct-current circuit formed of an exchange plant, telephone lines, and line terminal device. A direct-current resistance RS of the line terminal equipment viewed from the telephone line-side is the sum of the direct-current resistance RD of the bridge diode BD, a direct-current resistance RH of the hook switch HS and an input direct-current resistance RI of the DC-DC converter, satisfying the following equation, EQU RS=RD+RH+RI (1).
The direct-current resistance RS, in accordance with the JATE Japanese Approval Institute for Telecommunication Equipment standards, for example, is regulated at 50.OMEGA. to 300.OMEGA. n when the line current I is 20 mA to 120 mA. The direct-current resistance RS, in accordance with the FCC standards, for example, is regulated below 200.OMEGA. when the line current I is 30 mA to 120 mA. The line current I satisfies, EQU I=E/(R1+R2+RS) (2),
where, E represents a direct-current output voltage of the exchange plant, R1 a direct-current resistance of the exchange plant, and R2 a line resistance. The sum R1+R2, fluctuates depending on a length of the line, but normally is greater than
When the input direct-current resistance RI of the DC-DC converter is large, the DC-DC converter CN receives the direct-current power from the telephone line-side with a high efficiency.
However, the problem encountered in the conventional telephone line power utility circuit 501 is that, the receiving efficiency is low since the input direct-current resistance RI of the DC-DC converter CN is fixed at a lower value as a whole such that the direct-current resistance RS of the line terminal equipment viewed from the telephone line-side, takes a resistance value between 50.OMEGA. and 300.OMEGA..